An ink jet printer produces images on a receiver by ejecting ink droplets onto the receiver in a raster scanning fashion. The advantages of non-impact, low noise, low energy use, and low cost operation are largely responsible for the wide acceptance of ink jet printers in the marketplace.
Ink jet printers, however, may produce undesirable image defects in a printed image. One such image defect is non-uniform print density, such as “banding” and “streaking.” One major cause of “banding” and “streaking” is variation in the mass of the ink droplets ejected from different ink nozzles. These variations in ink mass may be caused by variations in the nozzles of a print head. The differences in the nozzles of a print head may be caused by deviations in the physical characteristics (e.g., the nozzle diameter, the channel width or length, etc.) or the electrical characteristics (e.g., thermal or mechanical activation power, etc.) of the nozzles. These variations are often introduced during print head manufacture and assembly.
The nozzles of a print head are typically arranged in arrays having row and columns. Therefore, banding and/or streaking effects may occur in a horizontal or vertical line of an image. The variations in the ink drops that cause these defects relate to the density, size, or morphology of the ink dots that form an image. These variations can have a static (i.e., consistent) component and a random (i.e., non-consistent) component. Random variations between ink dots are generally less visible because their effects tend to cancel-out each other. The static variations are usually repeated more consistently and, thus, are more likely to be visible as banding or streaking defects.
There are many techniques present in the prior art that describe methods of reducing banding artifacts caused by nozzle-to-nozzle differences using methods referred to as “interlacing,” “print masking,” or “multi-pass printing.” These techniques employ methods of advancing a media sheet or image drum by an increment less than the print head width, so that successive passes or swaths of the print head overlap. This type of control has the effect that neighboring image raster lines are printed using more than one nozzle. Therefore drop volume or drop trajectory errors observed in a given printed raster line are reduced because the nozzle-to-nozzle differences are averaged out as the neighboring nozzle mixing is increased. Other methods known in the art take advantage of multi-pass printing to reduce banding by using operative nozzles to compensate for failed or malfunctioning nozzles. For example, U.S. Pat. Nos. 6,354,689 and 6,273,542 to Couwenhoven et al., teach methods of correcting malfunctioning nozzles that have trajectory or drop volume errors in a multi-pass inkjet printer wherein other nozzles that print along substantially the same raster line as the malfunctioning nozzle are used instead of the malfunctioning nozzle. However, the above mentioned methods provide for reduced banding artifacts at the cost of increased print time, since the effective number of nozzles in the print head is reduced by a factor equal to the number of print passes.
Other techniques known in the art attempt to correct for drop volume variation by modifying the electrical signals that are used to activate the individual nozzles. For example, U.S. Pat. No. 6,428,134 to Clark et al. teaches a method of constructing waveforms for driving a piezoelectric inkjet print head to reduce ink drop volume variability. Similarly, U.S. Pat. No. 6,312,078 to Wen et al. teaches a method of reducing ink drop volume variability by modifying the drive voltage used to activate the nozzle.
Still other techniques known in the prior art address drop volume variation issues between print heads. For example, U.S. Pat. No. 6,154,227 to Lund teaches a method of adjusting the number of micro-drops printed in response to a drop volume parameter stored in programmable memory on the print head cartridge. This method reduces print density variation from print head to print head, but does not address print density variation from nozzle to nozzle within a print head. Also, U.S. Pat. Nos. 6,450,608 and 6,315,383 to Sarmast et al., teach methods of detecting inkjet nozzle trajectory errors and drop volume using a two-dimensional array of individual detectors.
One issue arising from variations in nozzle manufacture is the appearance of banding in the y-axis of an image. The y-axis of an image corresponds to the vertical dimension of an image. In an ink imaging device that ejects ink onto a media sheet, a banding defect may be seen in a line extending down the length of the page. In an ink imaging device that ejects ink onto a rotating image drum, a y-axis defect occurs in the direction of drum rotation. In some of the remedial techniques noted above, the driving signal to the nozzles of a print head are adjusted in response to measurements taken from a media sheet onto which a test image has been printed. These measurements typically include optical density measurements. Because an ink drop with a larger ink mass effectively absorbs more light than an ink drop having a smaller ink mass, measurements of the optical densities on a media sheet indicate which nozzles generate ink drops having large ink masses and those nozzles that generate ink drops having smaller ink masses. The voltage level of the driving signal may then be adjusted to reduce the mass of ink ejected by a nozzle producing too much ink or to increase the mass of ink ejected by a nozzle producing too little ink.
While these techniques may be useful in ink imaging devices that eject ink directly onto a media sheet or in an inkjet offset process, they may not be optimal or sufficient in ink imaging devices that scan the ink directly on the imaging surface. For example, in an offset process, the ink is ejected onto an intermediate drum prior to being transferred to paper. If done correctly, the above-described techniques enable field calibrations to be performed automatically by the printer to provide a better customer solution. Measuring jet-to-jet drop mass of ink on an intermediate transfer surface with an ink optical density sensor, however, is a challenging problem. Calibration time, cost, physical space constraints weigh against the use of a very sophisticated sensor. Also, most practical scanning systems have inherent sensor to sensor differences that add noise to the measurements. Other problems arise from the loss of information obtained from observing a printed test pattern on an intermediate transfer surface. For example, in an offset transfix process, such as the one described above, the ink spreads significantly during image transfer from the drum to the media. This spreading is achieved through a mechanical pressure process in which the nip between the transfer roller and the imaging drum presses the ink into the media sheet. Thus, larger drops spread out more than smaller drops with a resulting difference in intensity on the media. These intensity differences may be easily scanned and corrected. Another problem with jet-to-jet drop mass measurement on an intermediate transfer surface is the difference in contrast between the imaging drum and the ejected ink compared to the contrast achieved between ink and paper. Because the imaging drum is typically not as white and, therefore, not as reflective as a sheet of paper, for example, the optical density measurements of ink on an imaging drum are attenuated. Consequently, ink mass differences are more difficult to perceive from images on a rotating imaging drum. Therefore, methods of jet-to-jet calibration that increase or maximize the signal to noise ratio of the jet-to-jet drop mass are desirable.